1. Field of the Invention
This invention relates generally to apparatus and methods for creating and maintaining controlled mass transfer, heat transfer, or chemical reactions, and more particularly to an apparatus and method for contacting industrial volumes of gas and liquid phases in close microscopic scale proximity with each other for the purpose of creating and maintaining controlled mass transfer, heat transfer, or chemical reaction to achieve a particular process.
2. Brief Description of the Prior Art
A wide variety of industrial operations depend on contacting a gas and a liquid together to achieve a process. These operations can generally be categorized by transport phenomenon where two (2) or more components in a system having a gradient will naturally equilibrate. Gradients occur in the system due to differing concentrations, temperatures, or simply by differences in energy or motion between the components being brought into contact each with the other. Where the gradient is concentration, the transport is either by molecular mass transfer or convective mass transfer or both. In the cases where there are chemical interactions between the components, the transport equilibria may or may not be over shadowed by the driving force of chemical equilibrium. In general, these complex interactions which occur by contacting gas and liquid together can be categorized by the common terms mixing, stripping, evaporation, absorption, reaction, etc.
The literature dealing with mass transfer generally suggests that molecular mass transfer by diffusion plays a significant role even in a fast moving system regardless of whether the system is chemically reactive or non-reactive. Diffusion occurs across spatially separated components due to a natural tendency to equilibrate. Depending on the system of interest, the process may be limited by the diffusivity or by time. Diffusion may be vastly improved by minimizing the spatial distance between components. Contacting large volumes of gas and liquid together in conventional equipment generally involves creating the largest amount of liquid surface area by whatever means attainable in a limited volume. The result of this approach is most often a tall tower containing either trays or packing material where the liquid is sprayed into a gas phase or the gas is bubbled into the bulk liquid phase. In either case, the spatial separation between components is improved, but the overall volume required to accomplish the contact conditions on a commercial scale can be quite large.
An obvious parameter necessary for either approach to tower design is the acceleration of the components due to earth""s gravity. Tower bubble trays containing liquid rely on gravity to keep the liquid in the trays. Spray towers rely on gravity to accelerate liquid droplets downward. It is clear that neither method would work well in the absence of gravitational acceleration. For this reason and others, tower design is typically a large volume, low energy method for creating gas-liquid contactors.
Miller, U.S. Pat. Nos. 4,279,743; 4,397,741; 4,399,027; and Miller et al, U.S. Pat. Nos. 4,744,890 and 4,838,434 disclose air sparged hydrocyclone apparatus (ASH unit) primarily for use as a type of air flotation device for removing particulate matter from a liquid.
Atwood, U.S. Pat. No. 4,997,549 discloses an apparatus and method utilizing an air sparged hydrocyclone apparatus (ASH unit) for separating hydrophilic particles from a fluid suspension containing both hydrophilic and hydrophobic particles.
Grisham et al, U.S. Pat. Nos. 5,529,701; 5,531,904; 5,662,811; 5,730,875, and 6,004,386 disclose a compact, high energy apparatus and method for contacting gas and liquid (the group of patents being hereinafter referred to as the xe2x80x9cGrisham, et al Patentsxe2x80x9d). The xe2x80x9cGrisham, et al Patentsxe2x80x9d are based on accelerating a thin film of liquid in a helical flow pattern around and along the inside walls of a microscopically porous tube and sparging gas into the outside of the tube causing the gas to also pass through the thin liquid film. The goal central to these references is to decrease the diffusion distance between components by creating closely spaced gas bubbles in a fast moving liquid so that gas-liquid interfaces are abundantly available for mass transfer equilibrium to occur in a time as near instantaneously as is possible.
U.S. Pat. Nos. 5,529,701; 5,531,904; 5,662,811; 5,730,875, and 6,004,386, (the xe2x80x9cGrisham, et al Patentsxe2x80x9d), in which the inventor of the present invention was also a co-inventor, are hereby incorporated by reference to the same extent as if fully set forth herein.
Cairo, Jr. et al, U.S. Pat. No. 5,591,347 discloses a simplified single cell apparatus and method for removal of suspended impurities in liquids using gas flotation and filtration. The method and apparatus are preferably directed toward induced gas flotation separation of suspended impurities in combination with a filter media for filtration removal of remaining suspended impurities. A filter media is contained within the single cell apparatus such that liquid exiting the vessel must pass through the filter media after having been subjected to flotation treatment.
A foreign treatise written on a compact, high intensity gas/liquid contactor, xe2x80x9cStripping Performance of a New High Intensity Gas/Liquid Contactorxe2x80x9d, B. Waldie and W. K. Harris, Dept Mechanical and Chemical Engineering, Heriot-Watt University, Edinburgh, UK exists in the literature. A complete reference for this work is not known, but it appears to have been the culmination of a funded research supported by the UK EPSRC and several oil operating companies as part of an MTD programme on xe2x80x9cTreatment of Water Offshore-IIIxe2x80x9d. This paper deals with the comparative mass transfer performance of a laboratory device similar to both the Grisham, et al Patentsxe2x80x9d and ASH units and a small packed column. The results given were for HTU or xe2x80x98Height of Transfer Unitxe2x80x99 correlation whilst stripping toluene or oxygen from seawater and from fresh water. The conclusion states a 250-fold improvement in process performance of the compact device over a packed column.
In general, the prior art has taught a definite shift in thinking by the researchers and developers working in this field. The shift is recognition that liquid surface area can be increased by orders of magnitude over gravity dependent methods by containing liquid in an acceleration field and introducing gas into the acceleration field. The introduction of gas through porous media and further into the liquid provides a convenient way to control the gas bubble size by choosing beneficial porous media shape, porosity and permeability properties. It is desirable to obtain the smallest practical gas bubble size distribution flowing through a thin liquid film to achieve the largest liquid surface area per unit volume. A flat porous plate with a fast moving liquid film and introduction of gas from the underside of the plate would be a nice model to analyze mathematically, however, a cylindrical porous containment is more practical to build and offers better control over liquid film thickness and fluid dynamics in general. The result of a radial acceleration field is that it is, for all practical purposes, independent of its orientation with respect to earth""s gravitational field.
The Grisham et al. patents teach a device that is generally horizontally disposed relative to earth""s gravitational field. The Grisham et al. patents also teach a device that comprises at least one cylindrical porous tube that is coaxially aligned with a non-porous outer jacket, more particularly, a long porous element divided into segregated pressure chambers along the length of the porous tube. The porous element may be divided into two or more segments and mated together end to end to form a longer tube. The acceleration stated to operate the device is up to 1500 times the earth""s gravitational acceleration or 32.2 ft/s2 (g) or about 48,000 ft/s2 or as little as 400 g or roughly 13,000 ft/s2. The stated volumetric gas rate to volumetric liquid rate is up to 50 to 1 or as little as 10 to 1. It is unclear whether this volumetric gas flow rate is based on standard temperature and pressure conditions or actual temperature and pressure conditions. Since gas is compressible, the rates expressed in volumetric units can vary widely depending on the temperature and pressure at actual conditions. The stated number of liquid revolutions is up to 50 revolutions from the point of liquid introduction to the point of liquid exit from the device.
A great deal of effort and debate surrounds the length of porous media required in this invention and consequently, the residence time required to achieve the desired results in a particular process. Grisham et al, U.S. Pat. No. 5,529,701, column 13, paragraph 25, suggests that each incremental volume of liquid needs to reside for 0.5 seconds in the gas sparged acceleration field to achieve equilibrium. With this time parameter enforced in the design of the invention, it becomes more clear why the tube is so long, why the hydraulic energy and liquid acceleration is so large to move liquid to the end of a long tube, why the volumetric gas rate is so high to balance the liquid energy without wetting the porous media over relatively large tube external surface area, and finally, why different gas pressure chambers are required to get good gas flow distribution along the entire length of a long porous tube. In practice, in those applications where diffusion distance limits equilibrium, the time of contact is mostly irrelevant. In those applications where chemical reaction time drives the process given extremely short diffusion distances, the residence time has to be considered and consequently, the apparatus and method of operation must allow adequate residence time to complete the specific reaction desired. Observation of test results with the Grisham et al. invention yields surprising results about the required residence time for a process to occur. It is suggested here that the literature dealing with this subject assumes a quiescent system where diffusion distance increases the time required to achieve equilibrium. In this system, the real residence time required has yet to be discovered application by application and in every case appears to be less time than the best prediction made by those observers skilled in the art of operating the same process with conventional installed equipment. The porous tube implicated in the Grisham et al. invention is made excessively long by imposing a residence time parameter on the invention design and applied to processes limited by diffusion distance and not by time. If this parameter is relaxed based on empirical observation, the other operating parameters and the overall invention design can be reworked, changed, and extended accordingly.
In practice, the Grisham et al apparatus has been manufactured using standard xe2x80x98off-the-shelfxe2x80x99 piping or tubing components such as tees and flanges. The device size is referenced by non-limiting example to flow between 15 and 250 gallons per minute of liquid. The device is scalable and it is stated in U.S. Pat. No. 6,004,386 at the top of column 16, xe2x80x9cthere may be a practical limit of scale versus utilization of multiple units of apparatus to accommodate large flow rates.xe2x80x9d Since the basis of all Grisham et al. patents is for a xe2x80x9ccoaxially aligned cylindrical porous tube . . . and further including at least one nonporous outer jacket disposed concentric with said at least one porous tube . . . xe2x80x9d mated end to end, the scale-up to large process volumes would necessitate a very large diameter porous tube. Although this may or may not be physically possible to accomplish in practice, consider the following example applied to the Grisham et al. design and utilizing the design parameters disclosed in the patents:
Assume process flow rate of 3000 gallons per minute,
Assume centrifugal acceleration=1000 g adequate to maintain liquid film stability 1000 g=32,200 ft/s2=v2/r where v is velocity and r is porous tube radius,
A 20xe2x80x3 diameter porous tube is indicated based on area ratios of 1/3 liquid, 2/3 gas.
Solving for velocity gives v=163.8 ft/s
The liquid injection nozzle cross-sectional area indicated is about 5.87 sq. inches.
This example indicates a liquid flow nozzle having liquid velocity of about 164 ft/s, and a Reynolds Number for 60xc2x0 F. water=2.6xc3x97106.
The above example illustrates basic hydraulic principles, and references may be found in the literature for the treatment of incompressible viscous flow through pipes. Standard engineering practice normally places a practical limit of flow velocity for liquids through pipes of 15 ft/s and generally not to exceed 20 ft/s. Incompressible flow through nozzles is generally discussed for flow metering devices in which the flow rate is proportional to the pressure drop measured across a diameter contraction. For meter applications, the calibration range cited in numerous literature sources is limited to a Reynolds Number of about 1xc3x97106. Although the nozzle is not a flow meter, the design basis may be similarly considered for practical application. If the design basis of the nozzle is limited, and the limitation imposed is by the Reynolds Number, then the limit for centrifugal acceleration in the above example would become as follows:
Reynolds Number=1xc3x97106 
Velocity=62.7 ft/s
Centrifugal acceleration=393 ft/s2=12.2 g
Based on the recited parameters, 12.2 g would not be sufficient to operate the device.
This example illustrates only one problem of scalability of the Grisham et al. apparatus. There are numerous others. The practical application of incompressible flow through the invention nozzle must also take into account the presence of particulate material contained in the process liquid with some applications. Moving liquid at 164 ft/s where solids are present in the liquid flowing through the invention nozzle would quickly erode the nozzle. The alternative method available to scale the Grisham et al. invention for large capacity is to utilize multiple apparatus assemblies of smaller capacity in place of one large capacity assembly. Since the invention is for a xe2x80x9ccoaxially aligned cylindrical porous tube . . . and further including at least one nonporous outer jacket disposed concentric with said at least one porous tube . . . xe2x80x9d, the use of multiple assemblies of the invention would take the form of multiple tubes with concentric tube outer jackets. For clarification, the form of the prior art applied to large flows requires using multiple assemblies of individual pressure containing units arranged in parallel to split a large process flow into numerous smaller flows for distribution to each smaller invention assembly. This arrangement may or may not be practical. The application of multiple pressure containing invention assemblies may be driven by economics and not by engineering design. The economics quickly reduce to an accounting of the number of pipes, fittings, valves, meters, controls, and in general, all of the ancillary equipment required to operate the invention arranged in individual pressure containing assemblies for large capacity operation in an industrial setting.
The present invention is distinguished over the prior art in general, and these patents in particular by an apparatus and method for contacting large volumes of gas and liquid together on a microscopic scale for mass transfer or other transport processes where the contact between liquid and gas occurs at the interfaces of a multitude of gas bubbles. The apparatus includes a plurality of cylindrical or conical porous tube elements inside a pressure/vacuum vessel assembled in a bundle similar to a heat exchanger and terminating at each end in a tube sheet; a tangential nozzle for introducing thin film liquid flow into the inside diameter of each porous element in a helical flow pattern around and along the inside walls of the porous media; seals on each end of the porous media to segregate the gas flow from the liquid flow without first passing through the porous media and through the thin liquid film; an annular flow separator target nozzle at the second end of the porous section to divert liquid flow to a vessel and redirect the gas flow in a direction countercurrent to the liquid flow. The present method of contacting large volumes of gas and liquid for mass transfer or other transport processes in general comprises introducing a liquid flow tangentially into the inside diameter of each cylindrical or conical porous tube, assembled in a bundle and terminating at each end in a tube sheet, where the liquid flows in a thin film in a helical pattern around and along the inside walls of the porous media, controlling the hydrodynamics of the flow; sparging gas into the porous media and the thin liquid film at a proportional flow rate to the liquid flow rate so that an annular two phase flow with a uniform distribution of tiny gas bubbles results; maintaining a pressure balance and a distinct boundary layer near the inside diameter of each tube where the porous media does not become wetted by the flowing liquid; maintaining the annular thin film flow through the device so that enough energy remains to separate fluids due to differences in density at the exit of the device.
The present invention provides a method of creating and maintaining beneficial physical conditions for the transport of momentum, heat, and mass between a liquid and a gas across a multitude of microscopically small gas bubble interfaces so as to optimize the efficiency of interphase transport, and also provides economical modular apparatus for effective industrial scale utilization of the method. With the method and apparatus of the invention, interphase transfer equilibrium is achieved rapidly and within very compact theoretical unit volumes and physical apparatus volume due to greatly reduced spatial separation between components allowing diffusion to occur almost instantaneously.
The apparatus of the present invention generally includes a bundle of cylindrical or conical microscopically porous tubes, diametrically and circumferentially evenly spaced inside a pressure/vacuum vessel, oriented where the centerlines of each tube are parallel to the centerline of the vessel cylinder, terminating in perpendicular tube sheets similar to a shell and tube heat exchanger, sealed and seated at both ends in tube sheet tube seats, open to flow at both ends in the inside diameter of each tube, with porous walls and hollow interiors comprising each tube, liquid inlet nozzle assemblies disposed at the first ends of each tube, gas-liquid separator target nozzle assemblies disposed at the opposite second ends of each tube, a liquid collection pressure vessel, and a gas discharge assembly near the first ends of the tubes. The tubes are enclosed inside an outer vessel cylinder such that it forms one or more chambers capable of being pressurized with gas. The gas chamber may be commonly pressurized or it may be divided into multiple chambers or sections so that one or more tubes or groups of tubes may be pressure isolated from the other tubes in the bundle and distribution of gas to each tube or group of tubes may be controlled individually. The liquid collection pressure vessel may be oriented either vertically or horizontally depending on process requirements, preference and/or on the space available for a particular application.
Liquid is introduced tangentially into the inside diameter of each tube through liquid inlet nozzle assemblies with sufficient pressure and flow rate to create a high velocity flow of the liquid in a thin film around and along the inner surface of the porous wall of each tube. When the liquid meets the interior of a tube, the inlet velocity vector may or may not be divided into a radial velocity component vector and a longitudinal velocity component vector. The longitudinal velocity vector component may at first introduction of liquid be equal to zero. The addition of a longitudinal velocity vector component, if desired, may be accomplished by controlling the lead angle at which the liquid inlet nozzle is disposed tangentially relative to the inside diameter of the tube. Where the direction of longitudinal flow in the device is approximately in the same direction as earth""s gravitational acceleration vector, the lead angle is not particularly required to achieve the desired flow pattern. The high velocity flow of liquid in a helical pattern around and along the inside walls of the tube produces a centrifugal or outward force of sufficient magnitude, acting to force the liquid against the inner surface of the tube with a velocity vector direction generally normal or perpendicular to the longitudinal axis of the tube. The liquid radial velocity, and thus the outward acceleration, is sufficient to maintain the liquid film against the inner wall surface of the tube throughout its entire length.
In a first embodiment, pressurized liquid is introduced to all liquid inlet nozzle assemblies connected by a common pressure chamber at the same flow rate and pressure simultaneously. In a second embodiment, each inlet nozzle assembly has a separate liquid inlet and pressurized liquid is introduced to each porous tube individually. The liquid flow rate and pressure individually feeding each tube or group of tubes may be controlled such that one or more tubes or group of tubes may be turned off while other tubes or tube groups are still in operation. Liquid flow control in this configuration allows for sequential ranges or step-wise turn-up/turn-down so that overall a broader range of process turn-down ratio may be achieved. This arrangement also allows the use of tubes of differing diameters and capacities to cover the overall liquid capacity range and turndown ratio required by the process.
Pressurized gas is introduced into the pressure vessel chamber or chambers and forced through the porous walls of the tubes by virtue of the differential pressure between the pressurized gas chamber and the inside diameter zone of each porous tube. The tubes are seated and sealed in tube sheets such that the gas can only flow through the porous walls of the tubes. Where liquid flow control exists for each tube or group of tubes individually in the bundle, the gas supply to the same each tube or group of tubes may be individually controlled so that gas is not flowing to a tube or tube group that is out of service. The gas exits the porous wall at its inside diameter surface and is immediately contacted by the liquid, which is moving at high velocity relative to the tube wall and to the gas as it enters the interior of the tube. The gas is sheared from the porous wall by a liquid boundary layer moving approximately perpendicular relative to the gas. The result of this introduction of gas through the labyrinth of pores in the porous tubes into a liquid having a centrifugal or outward acceleration in the approximate opposite direction to the gas velocity vector direction is that a multitude of very fine bubbles are produced, and are carried away from the tube wall by the moving liquid in its radial flow pattern around the inside diameter surface of each porous wall, and longitudinally toward the liquid exit from each tube. The mixture of liquid and gas bubbles forms a two-phase flow that exists in a helical flow pattern around and along the inner surface of each tube. The buoyancy of the bubbles relative to the liquid causes them to move toward the region of lowest pressure or the center zone of the tube and against the centrifugal (outward) acceleration of the liquid phase, passing through the froth created by the two phase flow as it moves around the inner surface of each tube. The gas exits from the two-phase flow at the inner flow boundary created at the inside diameter of the thin film and is transported axially from the tube. Because the specific gravity of the liquid is much higher than the specific gravity of the gas, the centrifugal acceleration imposes a substantially higher force on the liquid than on the gas. The gas is thus able to move to the center of the tube while the liquid is forced toward the wall of the tube. The result of this density difference produces a distinct gas phase in and along the axial core of each porous tube, minimizing liquid entrainment with the gas in the central portion of the tube, and inducing a clean separation between the gas column at the center of the tube and the two phase flow along the inside diameter surface of the tube.
As the bubbles pass through the liquid, momentum, heat and mass are transferred on a molecular level between the liquid and the gas in accordance with the laws of thermodynamic equilibrium. Mass transfer occurs between the two components as determined by the value of the appropriate partition coefficient and the initial concentration of the transferring component in each phase. In general, the concentrations of the transferring component in each bubble of gas and in the immediately surrounding liquid are at or closely approaching equilibrium when the gas in each bubble exits from the liquid to the gas column at the center of the tube. Each volume of gas passes through the liquid only once within the apparatus, and each passage is associated with an approach to equilibrium.
The modular configuration of the present invention allows parameters to be adjusted for the most efficient operability range including broad turn-up/turn-down ratios that invariably have to be considered and incorporated. The modularity of the present invention also allows reduction of studies in process design, mechanical design, and cost accounting to determine the initial best tube diameter/length ratio versus tube number to roughly size for the operability range of the process. The process performance may be fine-tuned by simply adding or subtracting a tube from service. Tube porosity and tube length, liquid film thickness, gas outlet nozzle diameter, etc. can each be changed out in total or in selected contactors in the bundle.
Consolidating the gas-liquid contactor and associated hardware needed to house the porous tubes, collect liquid, scrub gas, etc. into a pressure/vacuum vessel allows further use of conventional vessel internal devices like baffles, mist eliminators, vanes, vortex breakers, etc. to improve the overall performance of the present invention. For example: gas baffles in the gas supply section of the vessel to prevent the incoming gas stream from impinging directly on the outside surface of a porous element and potentially creating uneven gas distribution to the entire available external surface area of the cylindrical or conical element; liquid motion baffles to mitigate sloshing for those process applications where the vessel is mounted on and operates on a moving deck, mesh pads and/or vane packs used in the gas scrubber section of the vessel to minimize any entrained liquid carryover prior to the final gas exit from the invention. Vessel external hardware commonly required to fabricate, install, start, and safely operate the process, and/or required by code may also be incorporated and would appear as vessel nozzles for level gauges/transmitters, extra vessel nozzles in general needed for gauges, meters, switches, vents/drains, pressure safety valves, etc.; along with required manways, davits, lifting lugs, code stamps, nameplates, etc.
The apparatus and method of the present invention, as well as the features and advantages associated therewith, will be described in more detail with reference to the accompanying drawing figures.